Concentrated acid hydrolysis of cellulose-containing material is effective on any substrate without pretreatment, can provide almost quantitative yields of fermentable glucose, affords less dilute solutions for fermentation, takes place in minutes and affords a relatively reactive lignin residue with potential by-product value. In contrast, neither dilute acid hydrolysis nor enzymatic hydrolysis can claim more than two of these attributes.
The critical drawback of concentrated acid hydrolysis processes has been the recovery cost of the acid. Most development work has been based on halogen acids because of their volatility. In an economic analysis of a hypothetical integrated wood hydrolysis plant using concentrated HCl it was determined that over 35 percent of the operating costs and almost 40 percent of the capital costs were related to HCl recovery and loss. See Nguyen et al., AIChE Symposium Series, 77:85-92 (1981). A more recent economic evaluation of concentrated halogen acid (including HCl and HF) hydrolysis processes, Wright et al., Evaluation of Concentrated Halogen Acid Hydrolysis Processes for Alcohol Fuel Production, SERI/TR-232-2386, Solar Energy Research Institute, Golden, Colo. (June 1985), confirmed that they are all expensive because of the high cost of acid recovery.
Although sulfuric acid is much cheaper than the halogen acids, it is not volatile and cannot be recovered by distillation. Even so, it was used in two uneconomical Japanese processes. In the Hokkaido process, described in Oshima, Wood Chemistry Process Engineering Aspects, Noyes Development, New York (1965), acid recovery depended on diffusion dialysis with an anion-exchange membrane. In the other process the sulfuric acid was neutralized with lime, and the resultant gypsum used for gypsum board manufacture. See Locke et al., Forest Products Journal 11:380 (1961).
Recent research has been directed at overcoming the recovery problem by the use of lower energy membrane separation technology. Simulation studies showed potentially large savings in recovery costs by electrodialysis. See Sklarewitz et al., Recycle of Hydrochloric Acid in a Wood Hydrolysis Plant by Membrane Technology, X InterAmerican Congress of Chemical Engineering, Santiago, Chile (Nov. 9, 1983); Sklarewitz, An Engineering and Economic Analysis for HCl Recovery by Electrodialysis in Concentrated Acid Wood Hydrolysis, North Carolina State University (1984). Ideally, the separation of HCl from the sugar in the hydrolyzate by electrodialysis should provide a maximum yield of recovered acid in maximum concentration at minimum power consumption with minimum membrane area; however, these conditions cannot be met simultaneously. At the highest current efficiencies and resultant lowest membrane area requirements, the final acid concentration in the concentrate solution is unacceptably low. At the highest final acid concentrations the percentage of acid transferred drops off and power consumption and membrane area are high. The present invention is a result of efforts to overcome these problems.